This invention relates to an adaptive control apparatus and, more particularly, to an adaptive control apparatus having a function for adjusting delay time so as to the null the delay time between a reference signal and a feedback signal.
In an apparatus for performing adaptive control using a feedback signal, the apparatus generally calculates the difference between a reference signal and a feedback signal, performs adaptive control so as to diminish the difference and produces an output upon reflecting the result of adaptive control in the reference signal.
FIG. 19 is a block diagram of an adaptive control apparatus 1 for compensating for distortion that occurs in a power amplifier. The apparatus includes an error calculation unit 1a for calculating the difference between a transmit signal x(t), which is a reference signal, and a feedback signal y(t); delay units 1b, 1c for delaying these signals so as to null the delay time between the reference signal and the feedback signal; and an adaptive controller 1d for performing adaptive control so as to diminish the error e(t) and for causing the result of adaptive control to be reflected in the reference signal. The signal obtained by reflecting the result of adaptive control in the reference signal is input to a modulator 2 by the adaptive controller 1d, the modulator 2 applies modulation processing to the input signal, a DA converter 3 converts the modulated signal to an analog signal and inputs the analog signal to a power amplifier 4, and the power amplifier 4 amplifies the input signal and then releases it into space from an antenna 5. A directional coupler 6 extracts part of the output of the power amplifier and inputs it to a demodulator 8 via an AD converter 7. The demodulator 8 applies demodulation processing to the input signal and inputs the demodulated signal to the adaptive control apparatus 1. The error calculation unit 1a calculates the difference e(t) between a reference signal x′(t) and a feedback signal y′(t) that enter via the delay units 1b, 1c, and the adaptive controller Id performs adaptive control so as to diminish the error e(t) and causes the result of adaptive control to be reflected in the next reference signal. By virtue of the control above, the feedback signal can be made to agree with a distortion-free reference signal. That is, the distortion produced by the power amplifier 4 can be compensated for and a distortion-free signal can be transmitted.
FIG. 20 is a diagram useful in describing the operation of the adaptive controller. The adaptive controller has a reflecting unit 11 for reflecting the result of adaptive control in the reference signal x(t) and outputting the resultant signal, and an adaptive control calculation unit 12. In the reflecting unit 11, a multiplier (predistortion unit) 11a multiplies the transmit signal x(t) by a distortion compensation coefficient hn(p) and outputs the result of multiplication. A distortion compensation coefficient memory 11b, which stores the distortion compensation coefficient hn(p) that conforms to power p [=|x(t)|2] of the transmit signal x(t), updates the distortion compensation coefficient hn(p) by a distortion compensation coefficient hn+1(p) found by an LMS adaptive algorithm in the adaptive control calculation unit 12. An address generator 11c calculates the power p [=|x(t)|2] of the transmit signal x(t) and outputs the result as a readout address AR. A delay unit 11d generates a write address AW of the distortion compensation coefficient memory 11b. The distortion compensation coefficient hn(p) is read out from the address AR indicated by the power p of the transmit signal x(t) and a prescribed period of time is required until the next distortion compensation coefficient hn+1(p) is found. Accordingly, the delay unit 11d is adapted so as to delay the generation of the write address for this period of time so that the old distortion compensation coefficient hn(p) can be updated by the new distortion compensation coefficient hn+1(p).
In the adaptive control calculation unit 12, a multiplier 12a performs multiplication between the difference signal e(t), which represents the difference between the reference signal x′(t) and the feedback signal y′(t), and a step-size parameter μ (<1), and a complex-conjugate signal output unit 12b outputs a complex conjugate signal y*(t). A multiplier 12c performs multiplication between the distortion compensation coefficient hn(p) and y*(t), and a multiplier 12d performs multiplication between μe(t) and u*(t). A delay unit 12e adjusts the timing at which the distortion compensation coefficient hn(p) is output, and an adder 12f adds the distortion compensation coefficient hn(p) and μe(t)u* and outputs the new distortion compensation coefficient hn+1(p).
The arithmetic operations performed by the arrangement set forth above are as set forth below. Here it is assumed that the power amplifier 4 has a distortion function f(p).hn+1(p)=hn(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=hn(p)x(t)f(p)u(t)=x(t)f(p)=h*n(p)y(t)P=|x(t)|2h*n(p)hn(p)=1where x, y, f, h, u, e represent complex numbers and * signifies a complex conjugate. By executing the processing set forth above, the distortion compensation coefficient hn(p) is updated and eventually converges to the optimum distortion compensation coefficient so that compensation is made for the distortion in the power amplifier 4.
Thus, it is necessary to equalize the delay time it takes for the reference signal x(t) to reach the error calculation unit 1a upon being input to the adaptive control apparatus 1 and the delay time it takes for the feedback signal y(t) to reach the error calculation unit 1a. In the prior art, therefore, a delay-time controller 9 is provided, as shown in FIG. 21, and controls the delay time of the delay unit 1c so that the timings at which the reference signal and feedback signal enter the error calculation unit 1a will agree. That is, while varying the delay time of the feedback signal, the delay-time controller 9 uses a correlator 9a to calculate the correlation between the reference signal output from the delay unit 1b and the feedback signal output from the delay unit 1c, and uses a delay-time setting unit 9b to obtain the delay time for which the correlation is maximized and to set this delay time in the delay unit 1c. As a result, it is possible to equalize the delay time it takes for the reference signal x(t) to reach the error calculation unit 1a upon being input to the adaptive control apparatus 1 and the delay time it takes for the feedback signal y(t) to reach the error calculation unit 1a. 
The delay time of the feedback signal varies depending upon the frequency band and bandwidth, etc., of the reference signal. No problems arise, therefore, if the frequency component of the reference signal x(t) is constant. If the frequency component varies, however, then the delay time until the reference signal x(t) arrives at the error calculation unit 1a and the delay time until the feedback signal y(t) arrives at the error calculation unit 1a will no longer be equal and adaptive control can no longer be performed favorably. For example, if the power amplifier is subjected to distortion compensation by adaptive control, the distortion compensation coefficient will not converge in optimum fashion and the noise floor will rise. It should be noted that even if the delay-time controller 9 controls delay time in real time, delay-time control will not be able to follow up the change in frequency and be executed at high speed, as a result of which the problems mentioned above will arise.
In a transmission apparatus that transmits a plurality of transmit signals using a multicarrier signal, the number of carriers and carrier placement vary depending upon whether or not each of the transmit signals is present, and the delay time of the feedback signal changes owing to this variation. The problems described above arise as a result.
Further, in multicarrier transmission, the delay time of the feedback signal varies also depending upon the power of each carrier signal and the temperature inside the apparatus. The problems described above arise as a result.